Embedded systems, e.g. a computer that is a component in a larger system and relies on its own microprocessor, are becoming commonplace in many items. Embedded systems are being used in personal electronic items such as PDAs, inkjet printers, cell phones, and car radios. Embedded systems are also becoming critical components of many industrial devices such as test and measurement systems, including the AGILENT TECHNOLOGIES J6802A and J6805A Distributed Network Analyzers.
To meet this growing demand, operating system providers, such as MICROSOFT, provide embedded versions of their normal operating systems. One of the more recent offerings from MICROSOFT is WINDOWS XP EMBEDDED (referred to herein as XPE). Embedded systems, such as XPE, provide functionality that recognizes the nature of embedded systems. One of the most important functions is to protect critical data, such as the operating system from being corrupted.
Embedded systems, much like personal computer systems generally store data in memory and/or mass storage. Mass storage may comprise, for example, a variety of removable and non-removable storage drives such as hard drives and compact flash media. Memory and mass storage components may be classified, in one respect, as either persistent or non-persistent. A persistent component stores data between boots of a computer system, whereas a non-persistent component does not (i.e., data stored in a non-persistent component is flushed between boots). In today's computer systems, memory is largely comprised of non-persistent components, and mass storage is largely comprised of persistent components.
Regardless of whether a component is persistent or non-persistent, the data stored therein is subject to corruption due to power surges, hard power downs, viruses, and so on. Although corrupted data may be cleared from non-persistent components by rebooting, it is much more difficult to remove corrupted data from persistent components--especially when the persistent component is the hard drive which stores a computer system's operating system. Corruption of operating system files can lead to intermittent failures of a computer system, or even worse, prevent the computer system from booting.
To prevent the corruption of critical data stored in an embedded system, it is desirable to write-protect the memory storing the critical data. If data cannot be written or overwritten to the memory, it is less likely that the data already stored thereon will become corrupted. The aforementioned XPE provides an enhanced write filter which, when enabled, intercepts all writes to designated portions of memory and redirects such writes either to another portion of memory (generally termed a write cache, but in XPE termed an “overlay”). The writes are cached in the write cache and made available as part of the protected memory. This gives the appearance that the protected portion of memory is writable while maintaining the integrity of the protected memory.
With the write filter enabled, corruption of the protected memory may be prevented. However, when the protected memory contains the operating system, it is necessary to allow some critical writes to be persisted to the protected memory. One example of a critical write is the changing of the computer's IP address. The enhanced write filter provides the capability of committing, e.g. writing, the data in the overlays to the protected memory. This action is also referred to as persisting. However, the XPE enhanced write filter simply commits all the data in the overlays and, as such, is unable to distinguish between critical writes and non-critical potentially damaging writes. Further, in the context of an embedded system the XPE fails to provide a safe environment for persisting data contained in the overlays.
Accordingly, the present inventors have recognized a need for apparatus and methods for controlling a write filter.